1. Field of the Invention
The present invention relates to a horizontal deflection output circuit for use in a television receiver or the like and, more particularly, is directed to a horizontal output circuit adapted for correction of horizontal linearity.
2. Description of the Prior Art
FIG. 1 shows a conventional horizontal output circuit employed for driving a horizontal deflecting coil in a cathode-ray tube (CRT) of a television receiver, and which includes a switching transistor T turned on and off synchronously with a horizontal periodic signal or drive pulse, a damper diode D, a resonance capacitor C.sub.R, a horizontal deflecting coil Ly, an S correcting capacitor C.sub.S, and a choke coil FBT normally constituted by a primary winding of a flyback transformer.
As is well known, such horizontal output circuit operates so that, when the switching transistor T is in its on-state, a sawtooth current is allowed to flow in the horizontal deflecting coil Ly. On the other hand, when the switching transistor T is turned off, the electromagnetic energy accumulated in the horizontal deflecting coil Ly is transferred to the resonance capacitor C.sub.R due to the resonance phenomenon arising from such coil in combination with the capacitor C.sub.R, and the resultant oscillation serves to invert the current in the horizontal deflecting coil. Then, the inverted current is decreased gradually via the damper diode D while charging the S correcting capacitor C.sub.S.
Accordingly, when the switching transistor T is turned on and off synchronously with the horizontal drive pulse, an alternating sawtooth current is made to flow in the horizontal deflecting coil Ly, and the resulting magnetic field causes an electron beam to scan in the horizontal direction in the CRT.
However, in such conventional horizontal output circuit, two factors are usually present which deteriorate the linearity of the horizontal scanning. The first factor is the difference between the center of curvature of the CRT fluorescent screen and the center of deflection of the electron beam, and such difference results in a raster distortion by which the image contracts at the center of the screen and expands at the opposite sides thereof.
The second factor is the deviation of the deflecting current from a straight line due to the series resistance existing in the output circuit (principally, the resistance of the deflecting coil Ly), whereby the deflecting current has a saturation curve with linear distortion such that the image expands at the left side of the screen and contracts at the right side of the screen.
In order to eliminate the raster distortion resulting from the first factor described above, it has been customary heretofore to form the deflecting current, for example, as indicated by the solid line 1 in FIG. 2(a), by utilizing the resonance of the S correcting capacitor so as to contract the image at the left and right hand sides relative to the center of the image. Further, for elimination of the linearity distortion caused by the second factor described above, it has been known to connect a saturable reactor having the inductance characteristic shown in FIG. 2(b) in series with the horizontal deflecting coil Ly so that the deflecting current has an overall characteristic represented by the curve 2, on FIG. 2(a), thereby emphasizing the contraction rate at the left side of the image while reducing the contraction rate at the right side thereof. Thus, the two kinds of horizontal distortion of the image are substantially eliminated by the conventional techniques mentioned above.
However, it is difficult to achieve complete correction of the linearity distortion by the provision of a saturable reactor (HLC), and different types of television receivers may require reactors with different characteristics leading to the further disadvantage of considerable variations due to the temperature characteristics, whereby it is extremely difficult to attain fine linearity correction. Accordingly, particularly with dimensional increases of the television screen, the beam scanning speed is not maintained constant at all horizontal positions across the image. In such case, when a caption is superimposed on the image by a projector so as to run horizontally across the displayed image, there may be a variation in the size of the characters running across the image, leading to difficulty in reading the caption.